The present invention relates to a method and apparatus for recovering heat from solid material discharged from hot processes such as combustion or gasification processes and/or from cleaning processes of hot gases. In particular, the invention relates to the reuse of recovered heat in hot processes, and more specifically, it is suitable for recovering heat from the ashes of gasification or combustion processes.
Handling of the ashes derived from the combustion and gasification processes is problematic. For one thing, the hot ash has to be cooled before storing, and for another, spreading of ash and especially the finest ash particles after cooling causes environmental hazards.
Efforts have been made to get rid of fine ash by agglomerating it by means of heating such either in a combustion chamber or in a separate agglomerating means, whereby the ash released from the process becomes more applicable to storage.
It has also been suggested to use ash for drying of fuel. For example, Swedish patent application 8501563-4 discloses mixing of hot ash with fuel prior to feeding the fuel into a combustion chamber. In this case, the moisture possibly contained in the fuel is either absorbed in the ash or evaporates. The fuel becomes drier and it is easier to handle in the equipment constructed for treating conventional dry material. At the same time, however, the quantity of ash circulating in the process becomes greater, which is less desirable in terms of energy economy.
Efforts have also been made to cool and agglomerate ash by mixing water therewith in order to receive ash which is more appropriate for storage, such as is disclosed in DE patent specification 31 01 847. In this disclosure, the cooling water constitutes a problem because the temperature of the water is low, it is difficult to utilize its heat. Depending on the method of cooling and amounts of water, the temperature of the cooling water of ash is generally below 200.degree. C. The water is too cold to be added in the steam circuit. On the other hand, the temperature of the cooling water is not much different from the temperature of the boiler water. Therefore, the cooling water cannot be used for heating of the boiler water with means of reasonable size. Discharging of water and not utilizing its heat is not a good solution either in terms of heat economy.
It is known from U.S. Pat. No. 4,244,706 a gasification process where both gaseous and solid products from the gasification are simultaneously cooled by direct heat exchange with water in a char cooling -device. Thus cooled product gas and vaporized water are separated from the solids and further cooled in order to condensate water and organic materials such as phenols and other aromatics, which are to be recycled. Cooling is accomplished by indirect heat transfer in a heat exchanger. The cooled condensate is mixed with fresh make-up water and thereafter mixed with coal in a mixing tank. The resulting mixture is preheated before being introduced into the gasifier.
It is known through U.S. Pat. No. 4,111,158 a method to cool bed material discharged from a fluidized bed reactor in order to control the bed temperature in the fluidized bed reactor. The bed material is discharged from the reactor into a heat exchanger where the material is cooled indirectly. The cooled bed material is thereafter recycled into the fluidized bed reactor.
The solids required for gasification and combustion processes have to be fed undisturbed into the combustion chamber and in such a manner that the various substances are distributed as evenly as possible over the cross sectional area of the entire combustion chamber. Feeding of dry, fine material, for example, into a pressurized combustion chamber calls for rather complicated equipment. Conveyance of dry material in the pipework prior to feeding it into the combustion chamber is both energy-consuming and difficult. For example, transfer of fine coal causes dusting and the fine coal itself is clearly explosive.
To avoid the above-mentioned drawbacks, moistening of coal prior to feeding thereof into the combustion chamber has been suggested. Finnish patent application 865217 discloses a method of mixing water with slack. Water is mixed with the slack to such an extent as to form a pumpable mass. This coal paste can be pumped with a pump without any additional treatment directly to the combustion stage.
An object of the present invention is to provide a method of recovering heat from solids discharged from hot processes, which method is superior in terms of the heat recovery to the methods described above. Another object of the invention is to provide an advantageous method of bringing the discharged solids into a form suitable for further treatment.
A still further object of the present invention is to provide an improved method of treating the feed material of the gasification or combustion process prior to feeding such feed material to a combustion or gasification reactor.
To gain the objects described above, the method of the invention, in which heat is recovered from hot solids separated from hot processes, includes following steps
solid feed material, such as solid fuel or additive for the process, and fluidizing gas, such as air is fed into the combustion chamber, PA0 process gas formed in the combustion chamber is discharged from the upper part of the combustion chamber and PA0 hot solid material, such as ash formed in the combustion chamber or other solid bed material being fluidized in the combustion chamber is PA0 hot water, thus formed in indirect heat exchange contact with substantially only discharged hot solid material, is conveyed uncooled from the heat exchanger through a conduit into a mixing chamber and mixed with solid feed material in the mixing chamber; PA0 the solid feed material is simultaneously heated and moistened by mixing it with the hot water in the mixing chamber and PA0 the heated and moistened solid feed material is conveyed from the mixing chamber through a conduit into the combustion chamber. PA0 a solid feed material conduit for feeding solids into the combustion chamber, PA0 fluidizing gas openings in the bottom grate of the combustion chamber for fluidizing solid material in the combustion chamber, PA0 a discharge pipe in the upper part of the combustion chamber, for discharging process gas from the combustion chamber, PA0 discharge means in the bottom part of the combustion chamber for discharging hot solid material from the combustion chamber and/or discharge means for discharging hot solid material separated from the hot process gases in a hot gas cleaning means, PA0 a heat exchanger connected to the discharge means for receiving the discharged hot solid material, PA0 a conduit in the heat exchanger for leading water through the heat exchanger in indirect heat exchange contact with the discharged hot solid material and for transferring heat from the hot solid material to the water, PA0 means for removing cooled solid material from the heat exchanger, PA0 a mixing chamber connected to the solid feed material conduit, PA0 means for introducing solid feed material into the mixing chamber and PA0 a conduit connecting the heat exchanger directly to the mixing chamber for feeding hot uncooled water from the heat exchanger into the mixing chamber for moisturing and heating the solid feed material in the mixing chamber.
discharged from the bottom part of the combustion chamber, through discharge means or
separated from process gas and discharged from the process in a hot gas cleaning means,
and conducted via a channel into a heat exchanger, where the hot solid material is brought into indirect heat exchange contact with water flowing through a conduit in the heat exchanger for transferring heat from the hot solid material into the water, thereby providing hot water,
The apparatus for recovering heat in accordance with the invention comprises
The present invention is suitable for recovering heat from the ashes of gasification or combustion processes, such as fly ash being continuously separated in the gas cleaning means. The invention is also suitable for recovering heat from bed material discharged from the combustion chambers of fluidized bed reactors. The bed material generally contains mainly ash and inert solid material such as sand. The bed material may also contain some additive, such as sulphur-binding calcium compounds, fed into the process. Bed material is removed either continuously or intermittently from the reactor for maintaining suitable process conditions in the reactor. Because the material to be discharged is hot, it usually has to be cooled prior to further treatment thereof.
In the method of the invention, the fluid used as a heat exchange medium is preferably water. Mixed with the feed material, it is well applicable to be fed into a hot process. Depending on the process and temperatures, some other fluid may also be used.
In accordance with the invention, hot solid material can be introduced either into an open fluid tank such as, for example, a water tank disposed below the combustion chamber of the reactor, or into a closed water tank. In a closed water tank, hot solid material effects vaporization of water, and it can be led forward in the evaporated form. Vapor is readily transported even over long distances. When vapor is brought into contact with colder feed material such as, for example, slack, it will condense onto the surface of the coal particles thus heating the coal and forming a coal paste suitable for feeding thereof into the combustion chamber.
Liquid is according to the invention brought into an indirect heat exchange contact with removed solid material.
When the invention is applied in respect of gasifiers or boilers in which coal is gasified or combusted, heated liquid or possibly steam is mixed with coal, preferably to such an extent as to produce coal paste which is easy to treat and handle. Increasing the moisture of coal to 15-50% facilitates its transportation by pumping even longer distances and enables simple feeding thereof even into a pressurized combustion chamber. Feeding can be effected by fairly simple means. Raising of the moisture content prevents dusting of coal and considerably reduces its susceptibility to explode.